1. Technical Field
This invention generally relates to semiconductor circuit design, and more specifically relates to fuse links in semiconductor circuits.
2. Background Art
The use of fusible links (or fuse links) is a known redundancy technique utilized in most large scale semiconductor devices such as dynamic random access memory (DRAM) and static random access memory (SRAM) products. Fuse links are used to access spare bit lines and/or word lines to increase yield potential. Typically, the semiconductor device is built and then tested. Some types of flaws in the device, such as bad bit lines in a DRAM, can be repaired by blowing an appropriate fuse link. The fuse links can be polysilicon or metal and can be blown by focusing an appropriate pulse of laser energy on the target fuse. When the laser strikes, the fuse link is vaporized and evaporated upward. Unfortunately, it has been observed that in some cases the fuse link is not completely opened after being struck with the laser pulse. This may result in enough leakage current passing through the partially blown fuse link to prevent successful repair of the semiconductor device.
Fuse links are also used in semiconductor identification techniques. For this use, individual fuse links are blown as a means of encoding identification information on the semiconductor device.
FIG. 1 is a cross sectional view of a fuse link 100 taken along line 1--1 of FIG. 2, where FIG. 2 is a schematic top view of a fuse link layout 200. The fuse link 100 is part of a portion of a semiconductor device 105. Fuse link 100 includes a fusible conductor 102, interconnects 112, and metal layer 114. Additionally, a fuse blow window 104 constitutes an area in which the oxide of the semiconductor device 105 is thinned to some predetermined thickness to allow the vaporized fuse material to be exploded upward through the fuse blow window 105. The long fusible conductor 102, typically on the order of 20 microns long, will usually constitute the next to last metal layer applied in the fabrication process (last metal minus one). Interconnects 112 serve to connect fusible conductor 102 to earlier metal layer 114 (last metal minus two), which connects to the rest of the semiconductor device.
Unfortunately, these prior art fuse inks 100 cannot be consistently blown with a single laser strike. One reason they are not consistently blown is that the conductor material of fusible conductor 102 at the edges of the laser strike region is melted, but not completely vaporized. The partially melted fusible conductors can then flow together, resulting in possible leakage current across fuse link 100. To overcome this inconsistency, the current process used to blow fuses incorporates two overlapping laser strikes to enhance the probability of completely opening the link.
FIG. 2 shows a schematic top view of a prior art fuse link layout 200, where fuse link layout 200 is typically a portion of a VLSI semiconductor device. Fuse link layout 200 includes a plurality of fuse in 100, a fuse window 104, and a plurality of tungsten barriers 106.
Fuse links 100 are typically blown with two laser strikes, the impact region of the two laser strikes being regions 108 and 110 illustrated in FIG. 2. Again, the two laser strikes are used to enhance the probability of completely opening the fuse link, and to prevent leakage from occurring. When two laser strikes are used, there is typically an area of overlap as illustrated in FIG. 2. Because of the overlap, a portion of the second laser pulse passes through the cavity created by the first, and does not stop until it is absorbed by the silicon substrate. In some cases, enough energy is absorbed by the substrate to cause damage to the silicon, which can propagate up and crack the overlying insulating layers. This damage can easily mean the semiconductor device.
The use of two laser strikes also increases the probability of a laser strike hitting outside the fuse blow window. Again, if sufficient energy is absorbed by the metal fuse under the thick oxide, cracks can result in the oxide which may propagate towards the surface or toward other fuse links. Additionally, the use of two laser strikes increases the time required, and hence reduces the production throughput.
The tungsten barriers 106 are used to protect adjacent fuse links from being damaged by the laser strike and its effects. The tungsten barriers are typically fabricated in the same process that creates interconnects between the last and next to last metal layers in the device. In processes where tungsten is not available, typically because a tapered via is used as the interconnect between the last and next to last metal layers, the fuse links 100 must be spaced farther apart to prevent damage. Typically, the fuse lines in a layout without tungsten barriers 106 must be spaced twice the normal distance. This results in significantly more area being required or far fewer fuse links being available.
Therefore, there existed a need to provide a fuse link for a semiconductor device that could be consistently blown with a single laser pulse. Additionally, there existed a need to provide a layout for these fuse links in a space-efficient manner.